Vehicle suspension including fluid communication circuit and accumulators

ABSTRACT

A vehicle having a body 25 supported respective pairs of front and rear wheels, 30, 31, 32, 33, two piston and cylinder units, 23-24, 35-36, 37-38, 34-39, being associated with each wheel and connected between the wheel 30 and body 25 so the piston moves in the cylinders in response to relative movement between the wheel and the body. An individual fluid circuit 40, 41, 42, 43, connecting one piston and cylinder unit 23 of each wheel 30 to a corresponding unit 34 longitudinally spaced therefrom on the same side of the vehicle, and the other piston and cylinder unit 24 of each wheel to a corresponding unit 35 on the transversely opposite side of the vehicle. This arrangement results in the fluid pressure in any two piston and cylinder units interconnected by an individual fluid circuit being equal, and relative movement between any one wheel and the vehicle body induces an opposite relative movement between the vehicle body and the wheel in fluid circuit therewith to maintain tractive ground engagement of all wheels.

This invention relates to a suspension system for a vehicle, and isspecifically related to controlling the movement of the wheels relativeto the vehicle body when traversing uneven surfaces and turning atspeed.

In known suspension systems resilient means such as springs or torsionbars are provided to perform a multiplicity of functions ranging fromthe absorption of impact loading (as from hitting bumps at speed) to theprovision of flexible support to enable all the wheels to maintaincontact with uneven terrain. Additionally applied loads such as cargodeflects traditional suspensions to induce wheel travel in a similarmanner to dynamic or contour loadings.

Traditional resiliently sprung suspensions are based on each wheelassembly being provided with an individual resilient component whichmechanically supports the respective "corners" of the vehicle. Theresilient components are rapidly progressive and normal vehicle weightis only distributed to each wheel when all the wheels collectivelydescribe a flat plane surface. When a wheel of a vehicle passes over (oris parked on) a bump, this wheel carries more vehiclular weight than itnormally carries on flat ground. Meanwhile the other wheels arecorrespondingly relieved of some of that weight.

These rapidly progressive resiliently sprung suspension systems worksatisfactorily only within a very narrow spectrum of dynamic, static andapplied loading situations, and any type of overloading or evenunderloading of a vehicle normally adversely affects its abilities tomaintain traction, average ground clearance, and quality of ride.Moreover the scope of demands upon known resilient suspension systemsleads to self conflicting performance characteristics as there is noinherent ability in the system to detect and react differently todiverse situations, which cause resonant rebounding, requiring excessivedamping with shock absorbers limiting free movement of unsprung weight.

Recently resilient spring suspension systems have begun to incorporatevariable damping and spring rates in an attempt to redress some of theshortcomings. Some other more advanced suspension systems (active andsemi-active suspensions) incorporate a number of electronic sensors andaccelerometers which monitor information such as vertical wheel traveland body roll, as well as speed, steering and braking commands. This andother data is processed by a computer which instructs hydraulicactuators to override the normal function of resilient springs in orderto interpret, compensate and adjust the suspensions performance to suitspeed, terrain and other factors in order to maintain a level ride andeven distribution of weight onto the wheels. These suspension systemstherefore need an external intelligent back-up system therefore need anexternal intelligent back-up system and the actuators require asubstantial input of external energy drawn from the vehicle engine.

A range of active and semi-active suspensions for vehicles have beenproposed recently including systems operating on the basis ofcompression and/or displacement of fluids and a limited number ofversions of these forms of suspension are already incorporated inproduction vehicles. However, the systems currently in use employing aliquid and/or gaseous medium usually incorporate a pump to maintain theliquid or gas at the required pressure and distribution, andsophisticated control mechanisms to regulate the operation of thesuspension system in accordance with sensed road and/or vehicleoperating conditions. These known systems incorporating pumps andelectronic control systems, are comparatively expensive to construct andmaintain and require energy input, and therefore have limitedacceptability in the vehicle industry.

It is therefore the object of the present invention to provide asuspension system which has the advantages of the liquid and/or gassystems, but is considerably simpler in construction and operates moreeffectively.

With this object in view, there is provided by the present invention avehicle having a load support body,

a pair of front ground engaging wheels and a pair of rear groundengaging wheels connected to the body to support same and each beingdisplaceable relative to the body in a generally vertical direction,

means interconnected between each wheel and the body including first andsecond fluid filled chambers that varies in volume in response tovertical movement between the respective wheel and the body,

respective first fluid communicating means connecting said firstchambers of the front and rear wheels on the same side of the vehicle toprovide respective individual fluid circuit between interconnectedchambers,

respective second fluid communicating means connecting the secondchambers of the front wheels and of the rear wheels respectively toprovide respective individual fluid circuits between interconnectedchambers,

whereby in use the fluid pressure in the two chambers of any individualfluid circuit is substantially the same thereby inducing all wheels tomaintain tractive ground engagement.

Conveniently at least one and preferably each of said individual fluidcircuits include at least one pressure accumulator means and preferablyalso a damping means operable to at least partially dissipate pressureshock in the fluid circuit. Under some vehicle operating conditions itcan be desirable to reduce or eliminate the operation of one or more ofthe pressure accumulator means, as further discussed herein, andaccordingly means can be provided to selectively restrict or terminatefluid flow to one or more of the pressure accumulatormeans, collectivelyor individually, from the respective fluid circuits.

Also it is convenient to provide means to selectively vary the rate orterminate fluid flow through one or more of the fluid circuits betweenthe chambers in one or both directions.

The vehicle suspension above described differs greatly from all theknown systems in that the wheel travel is not dependent upon progressiveresilient suspension mechanisms which require variable reactions to themany ever changing conditions experienced by the vehicle.

In the present proposal all wheels can freely follow even extremelyuneven terrain without changing the normal weight distribution onto allwheels, while also maintaining a substantially average vehicle bodyheight and inclination, and without limiting the extent of wheel travelmovements. Furthermore there is an unprecedented workinginterrelationship between wheels which are directly interconnected toeach other by the individual fluid circuits and collectively related tothe vehicle body so that resilient spring suspension means are onlynormally used to absorb and dampen dynamic shock, and do not inhibit thetranslation of wheel travel motions induced by uneven roads or terrain.

A vehicle supported on wheels in the above described manner allows freevertical travel of the individual wheels with respect to the vehiclebody or chassis without having to first overcome the resistance of theconventional springing mechanisms normally incorporated between thewheels and the vehicle body. Thus, there is provided a vehicle in whichthe wheels are individually unrestrained and free to move to follow theundulations of the surface being travelled without continually changingthe vehicle weight distribution between the individual wheels. Thisreduction or elimination of changes in weight distribution significantlyimproves the traction of the wheels to the surface being traversed andthe handling characteristics of the vehicle.

Furthermore, in known suspension systems the springs or like resilientdisplaceable mechanisms used are intended to both absorb dynamic forcesas well as permit non-dynamic wheel travel. However, in the vehiclesuspension now proposed, the resilient displaceable mechanisms may beoptionally omitted or temporarily deactivated whilst still allowingunrestricted wheel travel in a generally vertically direction, andmaintaining constant contact between the wheels and the ground, evenwhen travelling rough terrain.

The invention will be more readily understood from the followingdescription of one practical arrangement of one practical arrangement ofthe vehicle as illustrated in the accompanying drawings.

In the drawings,

FIG. 1 is a diagrammatic layout of a vehicle suspension;

FIG. 2 is a schematic plan view of a vehicle incorporating thesuspension system;

FIG. 3 is a schematic sectional view of suspension means and controlunit used in the suspension system;

FIG. 4 is a diagrammatic layout similar to FIG. 1 of a modification tothe suspension system.

FIG. 5 is a sectional view of the one construction of accumulator asused with double acting piston/cylinder units as referred to in relationto FIG. 1.

FIG. 1 shows in a diagrammatic form the basic layout of a vehicle wheelwhich operates a suspension system in accordance with the presentinvention. In this drawing the body or chassis of the vehicle is showndiagrammatically at 50 as a rectangular component, however, it is to beunderstood that the body or chassis can take a wide variety of forms andshapes depending on the particular construction and purpose of thevehicle. The body 50 is supported by four wheels comprising two frontsteerable wheels 51 and 52 and two rear non-steerable wheels 53 and 54.It is to be understood that the vehicle may also have all four wheelseither steerable or non-steerable, or the front wheels either steerableor non-steerable, or the front wheels may be fixed and the rear wheelssteerable, depending upon the particular requirements of any vehicle.Four double acting hydraulic cylinders or rams of basically conventionalconstruction are provided identified as 61, 62, 63 and 64. Each cylinderis pivotally connected to a respective wheel at one end and to the body50 at the other end so that it may pivot relative to the wheel and bodyabout respective parallel axes extending generally in the longitudinaldirection of the vehicle body. It is to be understood that additionalcomponents will be provided to connect the respective wheels to thevehicle body which will pivot relative to the wheel and body about axesparallel to the pivot axes of the cylinders, however, such additionalmembers have been omitted from the drawing for the sake of clarity.

Each of the double acting cylinders 61, 62, 63 and 64 are ofconventional construction having an outer housing with a cylindricalbore therein and a piston mounted in the bore with a piston rod coupledto the piston and extending through one end of the cylinder. A suitableseal is provided between the piston rod and the end of the cylinderthrough which it projects so that there is provided on each side of thepiston a fluid tight chamber with the volume of each chamber varying inresponse to movement of the piston in the cylinder. For the sake ofconvenience the chamber at the piston rod end of the cylinder shall beidentified as chamber A and that at the opposite end of the cylinder,that is, above the piston, will be identified as chamber B. Thus inrespect of cylinder 61, the piston rod end chamber shall hereinafter bereferred to as chamber 61A and the chamber above the piston will bereferred to as chamber 61B. The corresponding chambers in the othercylinders 62, 63 and 64 will be similarly identified.

The chambers 61B and 63B are interconnected by a fluid line 70 and thechambers 62B and 64B are interconnected by a fluid line 71. Similarlychamber 61A is connected to chamber 62A by the fluid line 73 and chamber63A and 64A are connected by line 74.

It is therefore seen that the respective chambers A of each wheel is indirect fluid communication with its corresponding chamber A associatedwith the wheel on the transversely opposite side of the vehicle, and therespective chambers B of each wheel are in direct communication with thecorresponding chambers B of the wheel at the opposite longitudinal endand on the same side of the vehicle. It will also be noted that thechambers at the piston rod end of the cylinders are connected to thecorresponding cylinder at the other wheel and the chamber above thepiston is connected to the corresponding chamber on the other wheel.Thus it will be appreciated that any variation in the capacity ofchamber A and B of the double acting cylinder associated with one wheelwill displace corresponding quantities of fluid from one double actingcylinder to the other two double acting cylinders connected thereto bythe fluid lines.

It will be understood that although the layout in FIG. 1 employs asingle double acting cylinder connected between each wheel and thevehicle chassis 50, the same operational result could be achieved byproviding two single acting cylinders connecting each wheel to thevehicle chassis, with the transverse fluid lines 73 and 74 connectingone cylinder of each wheel to the corresponding cylinder on thetransversely opposite wheel and the longitudinal fluid lines 70 and 71connecting the other cylinder from each wheel to the correspondingcylinder of the longitudinally spaced wheel on the same side of thevehicle.

With the cylinders arranged between the wheels and the vehicle chassisand interconnected as above described by fluid lines, a vehiclesuspension system is created whereby relative movement in a verticaldirection between any one wheel and the chassis 50 will result in acorresponding opposite movement between the transversely opposite wheeland the vehicle, and a corresponding movement between the longitudinallyspaced wheel and the vehicle on a corresponding same side of thevehicle. Expressed another way, vertical movement between a wheel andthe vehicle chassis in one direction, results in a similar verticalmovement between the diagonally opposite wheel and the chassis while theother two wheels will move in the opposite direction by a correspondingamount.

The result of this configuration of movements is that the vehiclechassis 50 will remain substantially level although its average heightwith respect to a selected ground datum may vary while all wheels remainin ground contact. It is also most important to note that in view of theinterconnection by the respective hydraulic lines of the four cylinders61, 62, 63 and 64, the pressures in the interconnected chamber of thefour double acting cylinders will be substantially the same. Thus theweight transferred from the chassis through the cylinders to each of thewheels will be substantially the same, whereby all of the wheels willremain in effective tractive engagement with the surface upon which thevehicle is supported or moving over.

Each of the fluid lines 70, 71, 73 and 74 are in communication withrespective hydraulic accumulators 75, 76, 77 and 200, and 78 and 201with a control or damping valve 80, 81, 82 and 208, and 83 and 209,interposed between the respective fluid line and accumulators. Eachaccumulator is divided in the known manner into two chambers by amovable internal wall. For convenience the chambers are designated C andD in each accumulator, compartment C being in communication with therespective fluid line and compartment D containing a compressed gas. Thehydraulic accumulators 75, 76, 77 and 78 as illustrated are the commonflexible diaphragm type, however, accumulators of other constructionsmay be used, including piston type, and accumulators using springs orother resilient mechanisms as a substitute for the compressed gascompartment. The accumulators 200 and 201 are of a specific constructionthat will be described in detail hereinafter.

When the valves 80, 81, 82, 83, 208 and 209 are open, the accumulatorsperform the primary function of providing a degree of resilience in thesuspension systems as during upward movement of any one wheel part ofthe displaced fluid in the associated cylinder can enter or leave thechamber C of the accumulator thus changing the amount of fluidtransferred to the interconnected cylinder and also compressing the gasin the chamber D, thereby increasing the pressure of the fluid in thefluid line interconnecting the two cylinders and hence also increasingthe pressure in the respective cylinder. When the vehicle is travellingin a generally straight line on an even surface, each of the controlvalves will normally be in an open position so as to provide a highlevel of resilience in the suspension system to thereby accommodateminor irregularities that may be encountered in the road surface withminimum vehicle body movement.

It will be noted that in the arrangement shown in FIG. 1, the fluid inlines 73 and 74 will normally be subjected to a below or sub-atmosphericpressure or suction when the vehicle is stationary or being supportednormally on its wheels. It is therefore preferable to include speciallyconstructed accumulators to operate with sub-atmospheric pressure in thelines, and which provide progressive resilience to increasingsub-atmospheric pressures. Accumulators 200 and 201 included in fluidlines 73 and 74 respectively are of this construction. Theseaccumulators may be of any known construction and one preferredconstruction is shown in FIG. 5.

Referring now to FIG. 5, the accumulator comprises a rigid houding 210with a cylindrical bore 211 and a dividing wall 212. The piston rod 207extends through the wall 212 in sealed sliding relation and rigidlyinterconnects the pistons 202 and 203 each in sealed sliding engagementwith the bore 211. The pistons form with dividing wall 212, the chamber215 which is vented to atmospheric by passage 213, and the chamber 206which is charged with a gas under pressure. The charging port 214 isprovided for connecting to a suitable gas recharging facility.

The piston 202 forms with the housing 210 the chamber 204 which in useis in communication with one of the fluid lines 73 or 74 via the controlvalve 208 or 209 as previously referred to.

In operation when the control valve is open and the sub-stmosphericpressure increases in the fluid line connected to the accumulator thepiston 202 is drawn downwardly in the chamber 204 as viewed in FIG. 5.This causes the piston 203 to also move downwardly in the chamber 206thereby compressing further the gas in chamber 206, until a balance isachieved between chambers 204 and 206, thus providing resilience to thesuspension system.

Under dynamic loading situations or when the wheels may be temporarilyrelieved of the vehicle weight the accumulators 77 and 78 provide theresilience to downward motions of the wheels relative to the body orchassis 50. The progressive shut-off valves 82 and 83, therefore, arenormally open when the lines 73 and 74 are positively pressurized andclosed when the lines are subject to sub-atmospheric pressures or insuction, while the shut-off valves 208 and 209 are normally open as theassociated lines are usually at sub-atmospheric pressure in mostoperating conditions.

In many circumstances accumulators 77 and 78 along with their valves 82and 83 may be totally omitted, as when known rubber stops areincorporated in the wheel assemblies to prevent bottoming out. It shouldbe understood that although the drawings show the transverse lines 73and 74 as being associated with the accumulators of the constructionshown in FIG. 5 and the chambers "A" of the piston/cylinders units, thelines between chamber "A" may equally well be located longitudinally ofthe vehicle between cylinders 61, 63, and 62, 64 respectively while thelines between chambers B may be located transversely between chambers ofcylinders 61, 62, 63 and 64.

In the case of this latter arrangement the accumulators 200 and 201would be in communication with longitudinal lines 70 and 71 and will inthis configuration provide roll control of the vehicle instead of pitchcontrol and resilience as shown in FIG. 1 as well as assisting withmaintaining vehicle height above ground.

When the vehicle is turning, particularly at a speed when significantcentrifugal forces are generated, the accumulator connected to the fluidline between the front and rear wheels on the outer side of the turningcircle is preferably isolated by closing the control valve associatedtherewith, whilst the accumulators on the opposite side of the vehicleand at the front and rear remain connected to the respective fluidlines. Thus as shown in FIG. 1 when the vehicle is turning to the left,the valve 81 would be closed to isolate the accumulator 76 from thefluid line 71 and the remaining valves 80, 82 and 83 remain open.

Under braking conditions, when there is a high dynamic load placed onthe front wheels, the valve 208, as seen in FIG. 1, would be closed toisolate the fluid line 73 from the accumulator 200, thus preventingdipping of the front of the vehicle. Similarly under acceleration when adynamic load is placed on the rear wheels, the valve 209 would be closedto prevent dipping of the rear of the vehicle. Closing of any such valvedoes not restrict normal articulation movement.

Suitable sensors can be provided on the vehicle to detect turning,braking and acceleration, and the signals from these sensors areprocessed through an ECU (electronic control unit) which controls theoperation of the valves 80, 81, 82 and 208, and 83 and 209. These valvescan be solenoid operated, preferably of a construction that permits thevalves to be opened to varying degrees to regulate the rate of flow offluid into and out of the accumulators. Thus the solenoid valves may inaddition to being opened and closed, may be set at any intermediateposition to control the rate of flow of fluid into and out of theaccumulators thus functioning as a variable damper.

It is to be understood that multiple accumulators may be provided incommunication with each of the fluid lines 70, 71, 73 and 74 with therespective accumulators on any one fluid line having an independentsolenoid valve controlling the communication between the accumulator andthe fluid line. Further, where more than one accumulator is provided ineach fluid line, the nominal pressure rating of each accumulator may bedifferent such that when the vehicle is operating under light loads alower pressure accumulator is used than when it is operating under highloads.

Also when cornering, such as to the left in FIG. 4, the low pressureaccumulator on the outer side of the vehicle (fluid line 71 in FIG. 1)is isolated and the high pressure accumulator on the inner side (fluidline 70 in FIG. 1) is isolated to minimize outward roll of the vehicle.

FIG. 2 of the drawings illustrates an alternative form of the suspensionsystem to that shown diagrammatically in FIG. 1. In FIG. 2, the vehiclechassis is shown in a more realistic form but is still to be consideredas fundamentally diagrammatic. In this drawing, each of the wheels, suchas the wheel 26, is connected to the chassis by a wishbone type arm 20which is pivotally connected to the chassis 25 by respective co-axialpivot connections 21 and 22. Further, the single double acting cylindersor rams, as described with respect to FIG. 1, connected between thevehicle chassis and the respective wheels, have each been replaced bytwo single acting cylinders 23 and 24, each pivotally connected to thechassis 25 at 27 and 28 and to the arm 20 at 29 and 30. The pivotconnections at the respective ends of the cylinders 23 and 24 arealigned in the generally longitudinal direction of the chassis 25whereby as the wheel 30 and the arm 20 carrying the wheel pivot relativeto the chassis 25, each of the cylinders 23 and 24 expand or retract.

The above description with respect to the mounting of the wheel 26 andthe interacting pair of cylinders 23 and 24 also applies to theconnection of each of the other three wheels 31, 32 and 33 of thevehicle, however, for the sake of clarity individual reference numeralsfor the corresponding components are not shown for each wheel mountingalthough the respective cylinders on each wheel have been individuallyidentified.

The cylinders 23 and 37 interacting respectively with the front wheels26 and 31 of the vehicle are interconnected by the fluid line 40 whilstthe cylinders 39 and 35 associated with rear wheels 32 and 33 areinterconnected by the fluid line 41. Similarly the cylinders 24 and 34associated with the front and rear wheels 26 and 33 respectively, areinterconnected by the fluid line 42, whilst the front and rear wheels 31and 32 on the opposite side of the vehicle are interconnected by thefluid line 43. Thus the operation of the respective pairs of singleacting cylinders associated with each wheel produce the identical effectin relation to the relative movement between the respective wheels ofthe vehicle and the vehicle chassis as has previously been describedwith respect to FIG. 1 wherein a single double acting cylinder isprovided between each wheel and the vehicle chassis.

The front and rear fluid lines 40 and 41 are in communication withrespective accumulators via individual damping valves as has previouslybeen described with respect to FIG. 1, however, the accumulators 45 and46, connected respectively to the fluid lines 42 and 43, as shown inFIG. 2 are provided with automatic damping or control valves 150 and 151in addition to damper valves 47 and 49 which functionally correspond todamper valves 80 and 81 in FIG. 1.

The automatic damper valves 150 and 151 are of identical constructionand each comprises a shuttle 152 shown in more detail in FIG. 3 axiallyslidable in a housing 154. The shuttle is of a stepped piston formhaving a small end 155 and a large end 156 with a transverse passage 157through the large end 156. The housing 154 also has a transverse passage158 which in use communicates with the accumulator 45 on one side andthe damping valve 47 on the other side. When the passages 152 and 158 inthe shuttle and housing respectively are in alignment, there is freepassage for fluid to pass between the damping valve 47 and theaccumulator 45 whilst axial displacement of the shuttle 152 in thehousing 154 in a downward direction as seen in FIG. 3 will progressivelyrestrict the flow passage between the damping valve 47 and theaccumulator 45.

The shuttle 152 and housing 154 are provided with respective shoulders160 and 161 arranged so that when the shoulder 61 of the shuttle engagesthe shoulder 161 of the housing, the transverse passage 157 in theshuttle will be in direct alignment with the transverse passage 158 ofthe housing so as to not restrict the flow through the passage 158.However, as the shuttle 152 is displaced from that position, thetransverse passage 158 in the housing 154 will be progressively reducedin the cross-sectional area thereby restricting the fluid flow to andfrom the accumulator 45.

As can be seen in FIGS. 2 and 3, the small end 155 of the shuttle 152 issubjected to the pressure in the compressible chamber 65 of theaccumulator 45 and the large end 156 of the shuttle 152 is subjected tothe pressure in the compressible chamber 66 of the accumulator 46. Thus,when the pressures in the chambers 65 and 66 are equal, the shuttle 152will be moved upwardly as seen in FIG. 3 so that the shoulders 160 and161 will abut and therefore the shuttle would offer no restriction ofthe flow between the damper valve 47 and the accumulator 45. However,when the pressure difference between the chambers 65 and 66 is such thatthe force applied to the small end 155 of the shuttle 152 is greaterthan the force applied to the large end 156 of the shuttle 152, then theshuttle will commence to move downwardly as seen in FIG. 3, therebyintroducing a restriction to the flow of fluid into or out of theaccumulator 45.

Under normal operating conditions when the pressure differential betweenthe accumulators 45 and 46 as a result of normal irregularities in thesurface being the pressure differentive will not be sufficient to causethe shuttle 153 to be displaced and thus it will not generate arestriction to the flow of fluid to or from the accumulator. However,under more severe conditions, such as when the vehicle is turning andthere is a substantial centrifugal force component applied to thevehicle wheels on the outer side of the turn, a sufficient pressuredifference will be developed between the accumulators 45 and 46 to closethe passage 158 in the housing of the automatic damper valve on theoutside of the vehicle whilst that on the inside of the vehicle willremain open. A spring, such as indicated at 163 in FIG. 3 may beprovided so as to achieve a progressive movement of the shuttle as thepressure differential increases thereby obtaining a progressive openingand closing of the passage 158.

FIG. 4 is a diagrammatic layout similar to that shown in FIG. 1 and likecomponents in FIG. 4 have been given the same reference numerals as inFIG. 1. In the construction shown in FIG. 4 there are two accumulatorsprovided in each of the fluid lines 70, 71, 73 and 74 identified as 90,91, 94, 95, 92, 93, 96 and 97 respectively with the respectiveaccumulators in each line having different nominal pressures. That is,in respect of line 70 the accumulator 91 has a higher pressure in thegas cavity 98 than in the gas cavity 99 in the accumulator 90. Each ofthe accumulators is provided with a respective control valve numbered100 to 107 which are solenoid or otherwise operated so that either oneor both of the accumulators associated with each line may be in activecommunication with that fluid line. Thus by way of example, as theaccumulator 90 has a lower nominal operating pressure than accumulator91, when the vehicle is operating in conditions requiring heavierspringing the accumulator 91 would be in communication with the fluidline 70 and the accumulator 90 isolated from the fluid line 70. Underlight springing requirement both accumulators may be in communicationwith the fluid line as accumulator 91 having the high operating pressurewill not function. The respective accumulators in each of the otherfluid lines would be similarly operated.

There is also provided in the configuration shown in FIG. 4 a shut-offvalve in each of the fluid lines, these shut off valves being designatedby the reference numerals 110 to 113, respectively. When the shut-offvalves are closed, it will be appreciated that there will be no transferof fluid or pressure from the cylinder associated with one wheels to thecylinder of the wheel transversely and longitudinally spaced therefrom.However, there will still be a degree of springing available through therespective accumulators where at least one accumulator will be incommunication with each end of each cylinder.

The advantage of providing the shut-off valves is that under high speedconditions when the vehicle is cornering it is desirable to limit theamount of movement between the front wheels and the chassis and to alesser extent between the rear wheels and the chassis.

Thus, when the vehicle is turning to the left as shown in FIG. 4,shut-off valves 110 and 111 will be closed, thus resisting the downwardmovement of the chassis relative to the wheel 52 arising from thecentrifugal forces developed during cornering. Also it would bepreferable for the low pressure accumulators 95 on the outer side of thevehicle, to be isolated from the fluid line 71, thus also restrictingthe movement of the body relative to the rear wheel 54 on the outer sideof the vehicle. Under the most severe cornering conditions at very highspeed, it would be preferable to also close shut-off valve 112 andisolate the accumulator 96 from the fluid line 74.

It will be appreciated that with the provision of the shut-off valves asabove discussed, and also valves between the respective accumulators andfluid lines, a high degree of control and variation in the performanceof the suspension system can be achieved particularly where therespective accumulators coupled to each fluid line have different loadratings, and those accumulators can be selectively coupled or de-coupledfrom the fluid line. Further, the control valves may be of a variablenature to provide a variable degree of damping between the respectiveaccumulators and the fluid lines.

Where the suspension system includes a range of controls to vary theperformance characteristics of the suspension as referred to above andelsewhere in this specification, it is convenient to provide anelectronic control unit (ECU) and a plurality of sensors to provideinput to the ECU which in turn will control the operation of the variousvalves, accumulators and dampers in the system to adjust the suspensionsystem to meet different vehicle operating conditions. In particular, asreferred to elsewhere in this specification, the characteristics of thesuspension system can be varied in response to acceleration, braking andturning of the vehicle, and appropriate sensors for detecting suchoperational conditions of the vehicle are known and used in othersuspension systems. Accordingly, details of such sensors and theinteraction thereof with ECUs shall not be described in further detailherein.

It is also to be understood that fixed or variable restrictors may beprovided in the fluid lines to control the rate of fluid movementthrough the lines between respective interconnected cylinders. A fixedrestriction may be achieved by variation in the bore of the fluid lineat one or more locations along its length. Also if the fluid used in thesystem was an electro rheological fluid, then an appropriate magneticfield generating device could be located on or in the fluid lines sothat the rate of fluid flow through those lines could be controlled byvarying the strength of the magnetic field.

In reference to FIGS. 2 and 3, the automatic damping valves 150 and 151are interconnected with the fluid lines running longitudinally of thevehicle and intercommunicating the cylinders of the front and rearaxles. However, it is to be understood that the same automatic dampingsystem can be incorporated between the front and rear transverse fluidlines such as fluid lines 40 and 41 in FIG. 2 so as to operate in a likemanner as described with respect to automatic dampers 150 and 151. Also,in the one suspension system, such automatic dampers may be providedboth between the longitudinal fluid lines and the transverse fluidlines.

It is also envisaged that a pump may be provided which may beselectively operated to transfer fluid from one side fluid line to theother, or alternatively from the front to the rear fluid lines and viceversa. Preferably a single pump may be used with a suitable switchableporting arrangement to connect the appropriate fluid lines to the pump.By transferring fluid between the front and rear fluid lines, there isprovided a control over the pitch trim of the vehicle whilsttransferring fluid between the respective side fluid lines to adjust theroll of the vehicle.

Also, the pump can be arranged so that it can be coupled to a fluidreservoir so that more or less fluid may be provided in the fluid linesthereby raising or lowering the nominal height of the vehicle body. Itis, however, to be understood that the provision of a pump to carry outthe above operations is not essential to the operation of the suspensionsystem, but merely provides additional capabilities of the system whichmay be used in connection with specific operating conditions of thevehicle.

Although the invention has been described herein with reference to atwo-axle vehicle, it will be readily appreciated that it may be appliedto a vehicle having multiple axles such as commonly referred to astandem axle assemblies. In a vehicle having a tandem wheel assembly eachwheel of the assembly is provided with two cylinders or a single doubleacting cylinder as previously described with reference to FIGS. 1 or 2.One cylinder of each assembly is connected to a common fluid lineextending longitudinally of the vehicle, one on each side of thevehicle. The other cylinder of each wheel is connected by an independentfluid line to the corresponding cylinder on the transversely oppositewheel.

Also it is to be understood that although not shown in the drawings aconventional resilient suspension element can be provided, such as aspring or torsion bar, connected between each wheel and the chassis.Where such a resilient suspension element is used it is preferablydesigned to support only the static weight of the suspended portion ofthe vehicle or a major portion thereof, the dynamic loading beingaccommodated by the fluid suspension hereinbefore described. Further itis to be understood that although the suspension system has been hereindescribed with reference to an hydraulic fluid being used in thecylinders and fluid lines, the system is equally operable with air orother gas as a substitute for the hydraulic fluid.

The claims defining the invention are as follows:
 1. A vehicle having aload support body,a pair of front ground engaging wheels and a pair ofrear ground engaging wheels connected to the body to support same andeach being displacable relative to the body in a generally verticaldirection, means interconnected between each wheel and the bodyincluding first and second fluid filled chambers that varies in volumein response to vertical movement between the respective wheel and thebody, respective first fluid communicating means connecting said firstchambers of the front and rear wheels on the same side of the vehicle toprovide respective individual fluid circuit between interconnectedchambers, respective second fluid communicating means connecting thesecond chambers of the front wheels and of the rear wheels respectivelyto provide respective individual fluid circuits between interconnectedchambers, whereby in use the fluid pressure in the two chambers of anyindividual fluid circuit is substantially the same thereby inducing allwheels to maintain tractive ground engagement, at least said two secondfluid communicating means each including respective main pressureaccumulator means, and control means operable in response to a selectedvehicle operating condition to vary the rate of flow of fluid to therespective main pressure accumulator means of at least the second fluidcommunicating means, said control means including a valve operable inresponse to the pressure differential between the two second fluidcommunication means, said valve being arranged to reduce the rate offlow of fluid to the main pressure accumulator of one second fluidcommunicating means in response to the pressure in that one second fluidcommunicating means being greater than the pressure in the other secondcommunicating means by a predetermined amount.
 2. A vehicle as claimedin claim 1 wherein the control means operable to vary the flow rate tothe main pressure accumulator means included in said two second fluidcommunicating means are operable to selectively prevent flow of fluid toany one of the main pressure accumulator means.
 3. A vehicle as claimedin claim 2, wherein said control means is operable in response tobraking of the vehicle to prevent flow of fluid to the main pressureaccumulator means communicated with the second fluid communicating meansinterconnecting the second chambers of the front wheels of the vehicle.4. A vehicle as claimed in claim 2 wherein said control means isoperable in response to acceleration of the vehicle to prevent flow offluid to the main pressure accumulator means communicated with thesecond fluid communicating means interconnecting the second chambers ofthe rear wheels of the vehicle.
 5. A vehicle as claimed in claim 1wherein said second fluid communicating means each include a furtherpressure accumulator means.
 6. A vehicle as claimed in claim 5, whereinsaid further pressure accumulator means are operable at differentpressure than that of the main pressure accumulator means of the secondfluid communicating means.
 7. A vehicle as claimed in claim 5, whereinfurther control means are provided operable in response to a selectedvehicle operating condition to vary the rate of flow of fluid to therespective further pressure accumulator means of at least the secondfluid communicating means.
 8. A vehicle as claimed in claim 1, whereinsaid two first fluid communicating means also each include a respectivemain pressure accumulator means.
 9. A vehicle as claimed in claim 8wherein the control means are operable to also selectively prevent flowof fluid to any of the main pressure accumulator means included in saidfirst fluid communicating means.
 10. A vehicle as claimed in claim 8,wherein said control means is operable in response to turning of thevehicle to prevent flow of fluid to the main pressure accumulator meanscommunicating with the first fluid communicating means connecting thefirst chambers of the front and rear wheels on the outer side of thevehicle with respect to the direction of turning.
 11. A vehicle asclaimed in claim 8, wherein at least said first fluid communicatingmeans each include respective further pressure accumulator means.
 12. Avehicle as claimed in claim 11, wherein said further pressureaccumulator means are operable at different pressures than that the mainpressure accumulator means of the first fluid communicating means.
 13. Avehicle as claimed in claim 11, wherein further control means areprovided operable in response to a selected vehicle operating conditionto vary the rate of flow of fluid to the respective further pressureaccumulator means of at least the first fluid communicating means.
 14. Avehicle as claimed in claim 1 including damping means operablyinterposed between each main pressure accumulator means and theremainder of the fluid communicating means to vary the rate of flow tothat main pressure accumulator means.
 15. A vehicle as claimed in claim5, including damping means operably interposed between each furtherpressure accumulator means and the remainder of the fluid communicatingmeans to vary the rate of flow to that further pressure accumulatormeans.
 16. A vehicle as claimed in claim 1 wherein at least one fluidcircuit includes means operable to selectively control the rate of flowof fluid between the chambers in that fluid circuit.
 17. A vehicle asclaimed in claim 1, wherein said valve is arranged to terminate the flowof fluid to main pressure accumulator of said one fluid communicatingmeans when the pressure in said one fluid communicating means is greaterthan the other by a predetermined maximum amount.
 18. A vehicle asclaimed in claim 9, wherein the control means include valve meansoperable in response to the pressure differential between the two firstfluid communication means, said valve means being arranged to reduce therate of flow of fluid to the main pressure accumulator means of onefirst fluid communicating means in response to the pressure in that onefirst fluid communicating means being greater than the pressure in theother first communicating means by a predetermined amount.
 19. A vehicleas claimed in claim 18, wherein said valve means is arranged toterminate the flow of fluid to the main pressure accumulator means ofsaid one first fluid communicating means when the pressure in said onefirst fluid communicating means is greater than that of the other, by apredetermined maximum amount.